A thermal contact enhancing interface material is a material that is positioned at the interface between two proximate surfaces for the purpose of improving the thermal contact between the two surfaces. This material is also known as a thermal interface material.
A fumed oxide is an oxide (e.g., a metal oxide) that has been produced through a gas-phase reaction, so that it is in the form of nanoparticles (size typically ranging from 5 nm to 500 nm (where nm=nanometer)) that are partly fused together to form a porous agglomerate. In contrast, a conventional oxide is not fumed, so that its particles are not fused together and are typically much larger than those in the range for nanoparticles. Throughout this document, oxides are not fumed, unless noted otherwise.
Due to the critical importance of microelectronics cooling to the reliability, performance and further miniaturization of computers and other microelectronic systems, the development of thermal interface materials is technologically important. Thermal interface materials are needed to improve thermal contacts, such as that between a microprocessor and a heat sink of a computer. As heat sinks improve, the bottleneck in the heat transfer shifts more and more to the thermal contact. This makes thermal interface materials increasingly important in the overall problem of thermal management in electronic systems.
The thicker is a thermal interface material, the higher is its thermal resistance in the direction of the thickness. Therefore, a small thickness is preferred. In order to attain a small thickness (ideally just enough to fill the valleys in the topography of the proximate surfaces), a thermal interface material is commonly in the form of a paste, which is known as a thermal paste. This paste comprises a base medium (i.e., the vehicle, which is a liquid) and a solid component (i.e., the filler). A small thickness can be attained if the paste is highly spreadable. Hence, spreadability is a preferred characteristic of a thermal paste.
A thermal paste displaces the air from the valleys of the topography of the proximate surfaces, which are never perfectly smooth. Since air is thermally insulating, this displacement results in an improved thermal contact. The ability to displace air hinges on the conformability of the thermal paste, i.e., the thermal paste needs to conform to the topography of the proximate surfaces. Since the topography is frequently in the micrometer scale (even finer than the micrometer scale), the paste needs to be able to fill valleys in a fine scale.
The solid component (also known as the filler) in a thermal paste is typically in the form of particles. The particles should be sufficiently small in size in order to fill the small valleys in the surface topography. Thus, nanoparticles are attractive for formulating thermal pastes.
It is preferable that a thermal paste is conductive thermally. The greater is the thickness of a thermal interface material, the more important is the thermal conductivity of the interface material. However, in case of a small thickness for the interface material, thermal conductivity is not very important, as long as it exceeds that of the air that it displaces.
The solid component is preferably nonconductive electrically. This is because of the possibility of seepage of the thermal paste from the thermal interface during transportation or use of the electronics. Seepage may cause contamination and, in case that the thermal paste is electrically conductive, it may cause undesirable electrical shortage as well.
A thermal interface material must withstand the elevated temperatures associated with the hotter of the two proximate surfaces that sandwich the interface material during use. In microelectronics, the elevated temperature is usually up to 100° C., although, in some applications, it can be up to 150° C. Therefore, thermal stability is another requirement of a thermal interface material.
After sitting for a sufficiently long time, a paste may undergo phase separation, i.e., the separation of the solid component of the paste from the vehicle. In most cases, the solid component tends to sink, due to gravity. Phase separation is not desirable. Therefore, a low tendency for phase separation is another attribute that is needed for a thermal paste.
Conformability, spreadability, electrical nonconductivity, thermal conductivity, thermal stability and phase separation resistance are attributes that are important for a high-performance thermal interface material. Inadequacy in any of these attributes will undermine the performance of a thermal interface material.
Thermal interface material of the prior art are commonly disadvantageous in their poor conformability and/or electrical conductivity. For example, silver pastes in the prior art (e.g., Arctic Silver 5, a product of Arctic Silver, Inc., Visalia, Calif.) are disadvantageous in both poor conformability (due to the high volume fraction of silver particles in the paste) and electrical conductivity (due to the high electrical conductivity of silver); carbon black pastes in the prior art (U.S. patent application Ser. No. 11/427,150 (2006), which is hereby incorporated by reference in its entirety) are disadvantageous in the electrical conductivity (due to the moderate electrical conductivity of carbon black), though they are highly conformable (due to the squishability or extensive compressibility of carbon black); ceramic (e.g., boron nitride) pastes in the prior art (e.g., Ceramique, a product of Arctic Silver, Inc., Visalia, Calif.) are disadvantageous in the poor conformability (due to the high volume fraction of ceramic particles in the paste), though they are electrically nonconductive (due to the electrical nonconductivity of ceramics).
The combination of thermal conductivity and electrical non-conductivity is not exhibited by most conductive materials. For example, metals and graphite are both electrically and thermally conductive. In contrast, polymers (other than those that have been specially doped) are nonconductive both electrically and thermally.
Diamond is particularly attractive in its combination of exceptionally high thermal conductivity and electrical nonconductivity, but it is expensive. Ceramics such as boron nitride (BN), zinc oxide (ZnO) and alumina (i.e., aluminum oxide, Al2O3) are not as thermally conductive as diamond, but they are thermally conductive to various degrees and are all electrically nonconductive, and are much less expensive than diamond. Among boron nitride, zinc oxide and alumina, boron nitride is the most conductive thermally, though it is the most expensive. Boron nitride is used as a thermally conductive constituent in thermal interface materials (U.S. 20040241410 (2004) and U.S. 20040081843 (2004), which are hereby incorporated by reference in their entirety). Compared to boron nitride, zinc oxide is less thermally conductive, but it is less expensive. Zinc oxide is the thermally conductive constituent of choice in the thermal interface material formulation in U.S. 6475962 (2002), which is hereby incorporated by reference in its entirety. Alumina is even less conductive thermally than zinc oxide, so it is not considered to be attractive for use in thermal interface materials.
The thermal conductivity within a thermal interface material should be distinguished from the thermal contact conductance across the thermal contact. It is the latter that describes the effectiveness of a thermal paste. Measurement of the latter requires measurement of the thermal resistance of the sandwich that consists of the two proximate surfaces and the thermal interface material between them. The sandwich includes the thermal interface material, the interface between the thermal interface material and one of the two proximate surfaces, and the interface between the thermal interface material and the other proximate surface.
Fumed oxides include zinc oxide, aluminum oxide, titanium dioxide, silicon dioxide, etc. Most fumed oxides are fumed metal oxides. Fumed oxides are formed by gas-phase reactions. An example of a method of producing a fumed oxide involves injecting a stream of a liquid feedstock containing a volatilizable oxide precursor into a stream of a combustion gas, with the temperature above the solidification temperature of the oxide particles (PCT Int. Appl. WO 2004048261 (2004), which is hereby incorporated by reference in its entirety). Among these oxides, zinc oxide is particularly attractive for its relatively high thermal conductivity.
Fumed oxides are used for (i) cosmetics that serve to disguise skin imperfections (U.S. Pat. Appl. US 2005163813 (2005), which is hereby incorporated by reference in its entirety), (ii) recording media with improved ozone resistance (PCT Int. Appl. WO 2004026766 (2004), which is hereby incorporated by reference in its entirety), (iii) catalytic converters for treating internal combustion engine exhaust (PCT Int. Appl. WO 2000057993 (2000), which is hereby incorporated by reference in its entirety), and (iv) abrasives for mechanical polishing (PCT Int. Appl. WO 9823697 (1998), which is hereby incorporated by reference in its entirety).
Fumed oxides such as fumed alumina have been previously disclosed for use as a minor solid additive (1-5 wt. %) in a thermally conductive paste that contains 60-90 wt. % of a highly conductive powder (such as silver, which is more conductive than alumina by orders of magnitude) (J. H. Mun and I. C. Sim, Republ. Korean Kongkae Taeho Kongbo KR 2002060926 (2002), which is hereby incorporated by reference in its entirety). Fumed oxides such as fumed alumina have also been disclosed for use as a minor solid additive (0.1-5.0%) in an electrically conductive paste that contains 15-60% of a highly conductive powder (such as silver) (B. M. Kim, Republ. Korean Kongkae Taeho Kongbo KR 2002061469 (2002), which is hereby incorporated by reference in its entirety). In the paste of Mun and Sim and the paste of Kim, the highly conductive powder (such as silver) is the major solid component and is the component that is responsible for the conductivity of the paste; the fumed oxide is not the component that is responsible for the conductivity of the paste. Fumed oxides have not been previously disclosed for use as the major conductive component in a conductive paste. Furthermore, no paste involving any fumed oxide in any proportion has been previously disclosed for use as a thermal interface material.
The conformability of a thermal paste depends not only on the solid component, but also on the vehicle, i.e., the matrix. A stiff matrix will result in poor conformability. Silicone is a soft and resilient matrix that is widely used for thermal interface materials. In spite of its softness and resilience, silicone exhibits high viscosity. Associated with the high viscosity is inadequacy in both conformability and spreadability. For example, U.S. Pat. Appl. Publ. US 20030171487 (2003) (which is hereby incorporated by reference in its entirety) uses silicone and recognizes the high viscosity of the resulting thermal interface material. U.S. Pat. Appl. Publ. US 20050150887 (2005) (which is hereby incorporated by reference in its entirety) also use silicone as the matrix.
During use, it is preferred that a thermal paste does not seep out of the interface, as the seepage can cause contamination and, in the case of an electrically conductive paste, short circuiting of the electronics around the thermal contact. Therefore, a thixotropic paste (a paste that flows only under an applied stress) is preferred to a fluidic paste (a paste that flows even in the absence of an applied stress). Silicone is thixotropic. Polyol ester can also be used to form a thixotropic paste, as described in U.S. Pat. No. 6,475,962 (2002) and U.S. Pat. Appl. Publ. US 20040018945 (2004), which are hereby incorporated by reference in their entirety.
The thermal stability of a paste is mostly governed by that of the vehicle, which is itself less thermally stable than the solid component. The choice and modification of the vehicle are typically used to improve the thermal stability of a paste. For example, the addition of one or more antioxidants to polyol ester can improve the thermal stability (Yasuhiro Aoyagi and D. D. L. Chung, “Effects of Antioxidants and the Solid Component on the Thermal Stability of Polyol-Ester-Based Thermal Pastes”, Journal of Materials Science 42(7), 2358-2375 (2007), which is hereby incorporated by reference in its entirety).
A paste is a suspension, which is a dispersion of fine particles in a liquid medium. The particles should be uniformly distributed, with little tendency of sinking or floating. The liquid may be aqueous (water-based) or non-aqueous. In this context, non-aqueous liquids include organic compounds and organometallic compounds, but do not include inorganic compounds. The type of liquid medium affects the ability to form a suspension, as different liquids interact with the solid particles differently. In addition, the chemistry of the solid surface affects the ability to form a suspension, as this chemistry affects the interaction of the solid with the liquid.
Aqueous suspensions are disadvantageous in the tendency for the water in the suspension to evaporate. Therefore, non-aqueous suspensions (i.e., suspensions that are not based on water) are attractive for applications in which the suspension needs to continue to exist for an extended period of time without the need for replenishment. In addition, water tends to promote corrosion, particularly the corrosion of metals. Examples of non-aqueous vehicles include oils (e.g., mineral oil), polyols (e.g., polyethylene glycol), polyol esters (e.g., dipentaerythritol, pentaerythritol and trimethylolpropane esters) and polysiloxanes (e.g., poly(dimethylsiloxane) and poly(diphenylsiloxane)).
A polyol (also known as polyhydric alcohol) is an alcohol having numerous hydroxyl groups. Polyols include polyethers, glycols, polyglycols, polyesters and polyglycerols. They constitute a class of organic materials that vary substantially in molecular shape, molecular length and melting temperature, thus providing choices that can suit the requirements of thermal interface materials.
Polyol esters are neopentyl polyol esters that are made by reacting monobasic fatty acids with polyhedric alcohols having a neopentyl structure. The neopentyl structure of polyol alcohols molecules is unique in that there are no hydrogens on the beta-carbon. As a result, polyol esters are usually characterized by relatively high polarity, relatively low volatility and relatively high lubricity, thus making them attractive for high temperature applications. Polyol esters are mainly used for jet engine lubricants and passenger car motor oils.
Polysiloxanes are polymerized siloxanes. Siloxanes are a class of organosilicon compounds with the empirical formula R2SiO, where R is an organic group.
Aqueous suspensions of fumed oxides are relevant to applications such as recording media and mechanical polishing. The art of making such suspensions has been reported (S. Gaydardzhiev and P. Ay, “Characterization of Aqueous Suspensions of Fumed Aluminium Oxide in Presence of Two Dolapix Dispersants”, Journal of Materials Science 41(16), 5257-5262 (2006); V. M. Gun'ko, V. I. Zarko, V. V. Turov, R. Leboda, E. Chibowski, E. M. Pakhlov, E. V. Goncharuk, M. Marciniak, E. F. Voronin and A. A. Chuiko, “Characterization of Fumed Alumina/Silica/Titania in the Gas Phase and in Aqueous Suspension”, Journal of Colloid and Interface Science 220(2), 302-323 (1999), which are hereby incorporated by reference in their entirety).
Applications other than those related to battery electrolytes have not been disclosed in relation to non-aqueous suspensions of fumed oxides. Non-aqueous suspensions in the form of gels of fumed silica in poly(ethylene glycol)dimethylether have been disclosed in relation to battery electrolyte applications (Yangxing Li and Peter S. Fedkiw, Rate Capabilities of Composite Gel Electrolytes Containing Fumed Silica Nanoparticles”, Journal of Electrochemical Society 153(11), A2126-A2132 (2006), which is hereby incorporated by reference in its entirety). Study of non-aqueous suspensions of fumed silica in poly(dimethyl siloxane) has been disclosed without consideration of any application (John Boyle, Ica Manas-Zloczower and Donald L. Feke, “Influence of Particle Morphology and Flow Conditions on the Dispersion Behavior of Fumed Silica in Silicone Polymers”, Part. Part. Syst. Charact. 21, 205-212 (2004), which is hereby incorporated by reference in its entirety).
The ability of the vehicle to wet the surface of the solid component in a paste is important for the dispersion of the solid particles in the vehicle and for the stability of the paste. Poor stability of a suspension means a substantial tendency for the solid particles in the suspension to separate from the vehicle. This separation is known as “phase separation”. An example of phase separation is the sinking of the particles in the paste, as in the case in which the particles have a higher density than the vehicle. Wetting means the ability of the vehicle to spread on the surface of the solid component. Poor wetting can result in the balling up of the vehicle on the solid component. The wettability depends on the energy of the interface between the solid component and the vehicle. A low energy is desirable for wettability.
Phase separation tends to occur in suspensions to various degrees. In other words, different suspensions can have different degrees of resistance to phase separation. A suspension with a lower resistance to phase separation will take a shorter time for the start of observable phase separation than a suspension with a high resistance to phase separation. Phase separation tends to occur with relatively high propensity when the surface of the solid component is not sufficiently wetted by the vehicle in the paste. Thus, wettability enhancement is typically used to improve the phase separation resistance.
The surface of oxides tends to exhibit inadequate wettability with organic vehicles, due to the hydrophilic nature of most oxides. In contrast, due to the hydrophobic character of carbon, carbon black tends to have good wettability with organic vehicles.
Appropriate treatment of the surface of a solid component may be useful for improving the wettability. For example, such a treatment provides certain functional groups to the surface of the solid component, thereby modifying the chemical behavior of the surface. The treatment of boron nitride with silane has been disclosed in relation to the preparation of an epoxy-matrix composite solid containing 44-57 vol. % boron nitride particles of particle size 5-11 μm (Yunsheng Xu and D. D. L. Chung, “Increasing the Thermal Conductivity of Boron Nitride and Aluminum Nitride Particle Epoxy-Matrix Composites by Particle Surface Treatment”, Composite Interfaces 7(4), 243-256 (2000), which is hereby incorporated by reference in its entirety). This treatment involves coating the boron nitride particles with a thin layer of silane. It is effective for improving the interface between the boron nitride particles and the epoxy matrix, so that the thermal conductivity of the resulting composite is increased. This composite is a monolithic solid; it is not a suspension.
A silane is a silicon analogue of an alkane hydrocarbon. It consists of a chain of silicon atoms covalently bound to hydrogen atoms. The general formula of a silane is SinH2n+2. Silanes are used as coupling agents. An example of an application is the use of silane to improve the adhesion of glass fibers with a polymer matrix in the fabrication of a glass fiber polymer-matrix composite.
Metal alloys with low melting temperatures (such as solders) applied in the molten state have long been used as thermal interface materials. However, they tend to suffer from the chemical reactivity of the liquid alloy with some metal surfaces (such as copper, which is commonly used as a heat sink material). Furthermore, alloys suffer from the need to be applied in the molten state and the need to heat in order to attain melting. In contrast, thermal pastes typically do not require heating.
The present invention is directed to overcoming these and other deficiencies in the art.